The present invention relates to an electrostatic coating blade for applying a thin layer of a liquid, e.g. oil, onto a target object; the present invention also provides a method of applying a coating of a liquid onto an object by elctrostatic spraying.
Electrostatic coating blades are well known for applying layers of paint or oil. One type of blade currently in use is made of metal and has a wedge shape that tapers to a discharge edge. A conduit extends longitudinally along the blade and a slot connects this conduit to the discharge edge for supplying liquid from the conduit to the discharge edge. When an electrostatic field of 50 to 140 kV is created between the object to be coated and the blade and when liquid, e.g. oil, is pumped along the conduit and through the slot, the field breaks up the liquid at the discharge edge into a number of conical streams which then in turn break up into charged droplets that are drawn by the field onto the object, which is thus covered in a thin liquid film. Using a blade of this type it is possible to achieve a minimum liquid discharge rate from the blade of approximately 0.5 ml/cm of blade per minute for a given oil but rates lower than this are not possible because, instead of steady conical streams, individual streams become intermittent which causes a discontinuous film on the object.
Attempts have been made to provide a uniform thin coating layer by limiting the amount of liquid fed to the discharge edge. One blade of this type is described in U.S. Pat. No. 2,695,002; the blade has a cylindrical body and a downwardly pointing lip extending along its length terminating in a discharge edge. A conduit extends along the length of the blade in which a rotor provided with a helical groove is located. As the rotor turns, liquid in the groove is fed into an outlet slot and from there the liquid flows onto the upper surface of the lip to form a thin stream that flows by the action of gravity to the discharge edge where it is discharged. The blade is usually made of steel but if the liquid is conductive, the blade may be made of an insulating material; however, the specification does not state how conductive a liquid must be to allow the blade to be made of insulating material. The width of the lip from the slot to the discharge edge is approximately 0.9 inches (23 mm). The minimum discharge rate of this blade necessary to produce a uniform coating on the target object is too high for the requirements of modern industry. Furthermore, since the blade relies on gravity to feed liquid from the slot to the discharge edge, the blade can only operate as a top blade, i.e. it can only coat objects located below it.
A further attempt to limit the amount of liquid reaching the discharge edge was to require liquid leaving a liquid outlet to flow over a surface towards the discharge edge under the action of gravity. A blade of this sort , which was produced commercially, is described in U.S. Pat. No. 3,486,483; the blade has a cylindrial body and a downwardly pointing lip that terminates in a discharge edge. The body is composed of an insulating material, while the lip has a sandwich construction with a conductive strip being located between two insulator layers; the edge of the strip is exposed near the discharge edge. The distance between the conductive strip and the discharge edge is approximately 10mm. A conduit extends along the length of the blade and exit holes are provided at the top of the cylindrical body so that liquid discharged from the exit holes flows over the outside of the body and onto the top surface of the lip; as the liquid stream flows over the cylindrical surface of the body and down the lip, it becomes thinner. When it reaches the discharge edge, the liquid stream is discharged at the discharge edge by virtue of the electrostatic field established between the object to be coated and the exposed edge of the conductive strip in the blade lip. However, the minimum discharge rate of this blade (while still producing a uniform coating on the target object) is still of the order of 0.5 ml/cm of blade length/minute; furthermore, since the flow of liquid between the outlet holes and the discharge edge depends on gravity, the blade can only be used as a top blade.
There is an increasing demand for a blade that can apply a thinner layer of liquid onto a target object while still requiring that the coating layer is continuous. This is particularly important in the steel industry where electrostatic coating blades are used to apply a layer of oil onto steel strip to prevent corrosion.
We have developed an electrostatic coating blade which has achieved application rates of oil as low as 0.03 ml/cm of blade length/per minute while still producing a uniform, continuous coating.
We have discovered that low discharge rates can be achieved by establishing an electrostatic field between the target object and the outlet(s) of one or more closed channels (by "closed" we mean that the channel has an inlet and an outlet but otherwise is not open to atmosphere) and placing an insulating surface in front of the channel outlets in such a way that a discharge edge provided at the end of the insulating surface is 0.5 to 4 mm from the channel outlets. In this way, liquid is drawn by the electrostatic field along the insulating surface in an ever tapering stream to the discharge edge and a very thin but uniform stream of liquid reaches the discharge edge where it is discharged evenly.